1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a liquid crystal display and a fabricating method thereof wherein a spacer is positioned for keeping a cell gap.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls the light transmittance of each liquid crystal cell in response to a video signal. Accordingly, a picture is displayed corresponding to the video signals on an LCD panel having liquid crystal cells arranged in a matrix. To this end, the LCD panel includes an active area having liquid crystal cells arranged in a matrix and driving circuits for driving the liquid crystal cells in the active area.
Referring to FIG. 1, a conventional LCD includes an upper plate consisting of a black matrix 20, a color filter 16, a common electrode 14 and an upper alignment film 12 that are sequentially provided on the upper substrate 11. The conventional LCD also includes a lower plate consisting of a lower substrate 1 on which a thin film transistor (TFT) 25/6/26/27/28/30, a pixel electrode 22 and a lower alignment film 10 are sequentially provided. In addition, the conventional LCD includes a spacer 24 and a liquid crystal (not shown) provided between the upper plate and the lower plate.
In the lower plate, the TFT includes a gate electrode 25 connected to a gate line (not shown), a source electrode 28 connected to a data line (not shown), and a drain electrode 30 connected, via a contact hole 23, to the pixel electrode 22. Further, the TFT includes a gate insulating film 6 for insulating the gate electrode 25, and an active semiconductor layer 26 on the gate insulating film 6 for creating a conductive channel between the source electrode 28 and the drain electrode 30 when a gate voltage is applied to the gate electrode 25. As shown in FIG. 1, ohmic semiconductor layers 27 are provided between the active layer 26 and the source/drain electrodes 28 and 30 and doped with an impurity for ohmic contacts between the active semiconductor layer 26 and the source electrode 28, and between the active semiconductor layer 26 and the drain electrode 30.
When a gate signal is applied to the gate line of the TFT, a data signal from a data line can be switched through the TFT to the pixel electrode 22. As a result, the liquid crystal is rotated by means of a voltage difference between a data signal applied to the pixel electrode 22 via the TFT and a common voltage Vcom applied to a common electrode 14. Accordingly, light transmission quantity through the liquid crystal is determined by the arrangement of the liquid crystal.
The pixel electrode 22 is positioned at an area adjacent to the intersection of a data line and a gate line, and is made from a transparent conductive material having a high light transmittance. The pixel electrode 22 is provided on a protective film 8 that is on the surface of the lower substrate 1, and is electrically connected, via a contact hole 23 in the protective film 8, to the drain electrode 30. An upper portion of the lower substrate 1, provided with the pixel electrode 22, is coated with an alignment film 10 that is subjected to a rubbing process, which completes the assembly of the lower plate.
The black matrix 20 of the upper plate is formed on the upper substrate 11 in correspondence with the TFT area of the lower plate and an area adjacent to the intersection of a gate line and a data line. The black matrix 20 also defines a liquid crystal cell area in which a color filter 16 will be formed. Further, the black matrix 20 plays a role in preventing light leakage and absorbing an external light such that contrast can be enhanced. The color filter 16 is formed in the cell area as defined by the black matrix 20. The color filter 16 specifically transmits a wavelength of light for a certain color, such as red, green or blue. The common electrode 14 is formed on the color filter 16. An indium-tin-oxide (ITO) with good light transmittance is used as the common electrode 14. Typically, indium-tin-oxide has a hydrostatic property of being hydrophobic. The alignment film 12 is formed by coating an alignment material, such as polyimide, on the common electrode 14 and by performing a rubbing process thereon.
Ball spacers, like ball spacer 24, are sprinkled onto either one of the upper plate or the lower plate of the LCD panel by means of a jet nozzle to keep a gap between the upper plate and the lower plate. The ball spacers should be uniformly distributed for the purpose of keeping a uniform cell gap of the LCD panel. However, it is difficult to uniformly distribute ball spacers due to the randomness in any sprinkling system. If the ball spacers are not uniformly distributed in the LCD panel, the cell gap in individual liquid crystal cells may not be uniform such that a stain-like appearance phenomenon is created in one or more areas on the LCD panel. In addition, if a user applies a pressure to the screen at the exterior of the LCD panel when ball spacers are used, a ripple phenomenon can occur in which the picture on the LCD panel has darkened areas shaped like waves. The darkened wave-shaped areas occur because the ball spacers have been shifted around between the upper plate and the lower plate.
Recently, there has been a study to provide a spacer that is fixed and patterned at a specific location to overcome the disadvantages of the ball spacer 24 and its sprinkling system. Hereinafter, a manufacturing method of the pattern spacer will be described with reference to FIGS. 2A to 2C and FIG. 3. More particularly, FIGS. 2A to 2C are cross-sectional views showing a process of manufacturing a conventional pattern spacer, which will be described in conjunction with FIG. 3, which is a flow chart.
A spacer material 42a, as shown in FIG. 2A is coated onto a substrate 40, as referred to in step S31 of FIG. 3. The substrate 40 can be either one of the upper plate or the lower plate provided with the TFT. The spacer material 42a is a material that is mixed with a solvent, a binder, a monomer and a photo-initiator. As referred to in step S32 of FIG. 3, the spacer material 42a is subject to a pre-curing to eliminate a solvent within the spacer material 42a, thereby making the spacer material 42a into a paste-like state.
Subsequently, as shown in FIG. 2B, a photo mask 44 having a transmission part 44a and a shielding part 44b is aligned on the spacer material 42a. As referred to in step S33 of FIG. 3, when an ultraviolet (UV) ray is radiated onto the spacer material 42a through the photo mask 44, the spacer material corresponding to the transmission part 44a is exposed to the ultraviolet ray.
As shown in FIG. 2C and referred to in step S34 of FIG. 3, the spacer material 42a is patterned. When the spacer material 42a is developed using a negative process, the spacer material 42a that is not exposed to the ultraviolet ray is removed while the spacer material that is exposed to the ultraviolet ray is left. When the spacer material 42a is developed using a positive process, the spacer material 42a that is exposed to the ultraviolet ray is removed while the spacer material that is not exposed to the ultraviolet ray is left. As referred to in step S35 of FIG. 3, the spacer material 42a patterned in this manner is cured to form spacers 42 having a desired height.
The spacers 42 for keeping a cell gap in the LCD can occupy about 20% of the entire area in a liquid crystal cell. If the spacer 42 is formed by the above-mentioned photolithography technique, then more than 95% of the coated spacer material 42a is wasted as a result of a spin-coating process of the photolithographic spacer material. Thus, the conventional photolithography wastes a lot of material in forming the spacer 42 and is inconvenient in that it requires a complex five-step process.
To reduce the waste of material and the number of process step, there has been a spacer formation method suggested using an ink-jet device as shown in FIGS. 4A to 4C. As shown in FIG. 4A, an ink-jet device 50 is aligned to correspond to a formation position of the spacer 58. In this alignment state, ink from the ink-jet device 50 is jetted to the substrate 40. The substrate 40 corresponds to at least one of the upper and lower plates of a LCD panel. The ink-jet device 50 jets ink using a thermal system or a piezoelectric system. Typically, the latter system is used. The ink-jet device 50 using the piezoelectric system consists of a vessel 52 for containing a material to be jetted, and an inkjet head 54 for jetting a material from the vessel 52.
The vessel 52 is filled with the spacer material 58, and the ink-jet head 54 is provided with a piezoelectric device and a nozzle 56 for jetting the spacer material 58 from the vessel 52. When a voltage is applied to the piezoelectric device, then a physical pressure is generated to cause a capillary phenomenon in which a flow path between the vessel 52 and the nozzle 56 repeatedly contracts and relaxes. Due to this capillary phenomenon, the spacer material 58 jets out of the nozzle 56 onto the substrate 40, as shown in FIG. 4A. Then, a curing process is used that exposes the spacer material 58 on the substrate 40 to an ultraviolet ray from a light source (not shown). Thus, the spacer material 58 can be hardened into a spacer 59, as shown in FIG. 4B.
Since the spacer material 58 has a hydrostatic property of being hydrophilic, the spacer 49 does not have good adhesion with the common electrode 14 that is also hydrophobic. Although the spacer may have a minimum height for keeping the cell gaps between the upper plate and the lower plate of the LCD panel, the lack of adhesion between the spacer and a plate of the LCD panel enables the spacers to move around.